びびってんぞ is short for びびっているぞ and びびる is a slang word that means to get scared or frightened. That phrase as a whole means the pitcher is getting nervous or his confidence is dropping. Apparently it's not uncommon among middle school and high school baseball players and some kids yell it out just to mount pressure on the pitcher on the opposing team but I've never heard anyone say it in a professional game, presumably because it sounds a little childish taunt. But then I have never played baseball in Japan.

【subject】ピッチャー（　pitcher　 ）　（が） 【dictionary meaning】びびる　become nervous／　get cold feet　　　　　　　 【current】 te-form ＋iru →　びびっている　【casual】びびっている→びびってる　【emfasis,remind】びびってる　＋　ぞ　→びびってるぞ　【euphonic change】びびってるぞ→びびってんぞー